The present invention involves a process for the purification treatment of liquid reaction and washing media obtained in the preparation of cellulose ethers, after organic solvents and/or other low-boiling organic components have been separated by distillation.
At present, cellulose ethers are frequently prepared by employing organic solvents, such as isopropanol, tertiary-butanol, acetone, dimethoxyethane or mixtures thereof, as dispersing auxiliaries in the process, as early as the alkalization step or not later than the etherification step. This applies, in particular, to the preparation of sodium carboxymethyl cellulose (NaCMC) or hydroxyethyl cellulose (HEC), which are intended to have the advantage of a particularly uniform substitution. To an increasing extent, also discussed is the use of organic solvents of this kind for the preparation of alkyl-hydroxyalkyl cellulose, for example, methyl-hydroxyethyl cellulose (MHEC).
Organic solvents, such as isopropanol or methanol, are likewise used in the treatment of crude (i.e., still unpurified) cellulose ethers, for example, in order to remove salts or other by-products of the reaction from the valuable cellulose ether product. As described in "Ullmanns Encyklopaedie der technischen Chemie" (Ullmann's Encyclopedia of Industrial Chemistry), Verlag Chemie-Weinheim 4th edition, volume 9, 1975, pages 192-212, keyword "Celluloseaether" (cellulose ethers), the etherification of cellulose is, however, accompanied by side-reactions (see, for example, page 201, right-hand column; page 202, left-hand column; page 205, left-hand column and page 206, right-hand column.) For example, these side-reactions include competitive reactions to the intended etherification resulting from hydrolysis of the etherifying agent or from the substitution of OH groups of the organic solvent or of by-products by the etherifying agent. These competitive reactions result in undesirable by-products. By-products which are obtained include, for example, methanol and dimethyl ether in the methylating reaction with CH.sub.3 Cl, glycolate ions (hydroxyacetate ions) in the carboxymethylation with monochloroacetic acid or the salts thereof, or ethylene glycol and polyethylene glycols in the hydroxyalkylation with alkylene oxides.
In view of the increasing importance of regulations mandating the least possible pollution of the environment by waste water, it is necessary to optimally purify waste waters before they are discharged into water systems. This applies particularly to waste waters containing organic by-products from chemical reactions, which have a high chemical oxygen demand (COD). In many cases, purification can already successfully be achieved by distillation of such liquid reaction and/or washing media; however, distillation only removes organic solvents, excess amounts of starting materials used, or by-products of the reaction, such as CH.sub.3 Cl, ethylene oxide, methanol, dimethyl ether or isopropanol, which are rather low-boiling, whereas high-boiling or non-volatile organic compounds remain in the distillation residue and consequently also in the waste water. Therefore, a very intensive search is being made for separation and purification processes which make possible the treatment of by-products of reactions, liquid reaction media and/or liquid washing media. In the prior art, the following processes have been disclosed in this connection:
In the process for producing alkali metal CMC according to German Offenlegungsschrift No. 1,518,576 (equivalent to U.S. Pat. No. 3,347,855), a water/acetone mixture is employed as the liquid medium during the alkalization and the etherification of the cellulose. After the reaction, the water/acetone mixture is separated from solid reaction products in a separator and transferred to an acetone recovery column. In this column, excess water is removed from the water/acetone mixture. The acetone is returned to the reaction cycle. No information is to be found concerning the treatment of the by-products of the reaction. Distillation is also mentioned in the description contained in Ullmann's Encyclopedia, referred to above, but this publication does not give any information concerning the treatment of by-products.
German Offenlegungsschrift No. 2,636,935 (equivalent to U.S Pat. No. 4,015,067) discloses a continuous process for preparing cellulose ethers, in which purification is carried out in such a way that the crude product is freed from its low-boiling constituents (by-products and excess etherifying agents) by steam distillation. These constituents can then be fractionated in a further still and, if desired, returned to the etherification process. As regards the treatment of the distillation residue obtained in each case, it is stated only that the residue from steam distillation (i.e., the crude cellulose ether which has been freed from low-boiling constituents) is filtered, washed, dried and ground.
According to German Offenlegungsschrift No. 3,118,915, the purification process for aqueous distillation residues produced during the treatment of liquids obtained in the preparation of carboxymethyl polysaccharides comprises separating these distillation residues into their components in ion exchangers. A process of this kind is, however, unsuitable in the case of distillation residues which result from production processes used for preparing non-ionic or mainly non-ionic cellulose ethers (particularly alkylhydroxyalkyl celluloses), since in that case compounds which are predominantly non-ionic have to be removed.
German Pat. No. 681,625 describes a process in which a cellulose ether, which is actually water-insoluble (MC or NaCMC), is first dissolved in an aqueous NaOH solution and is thereafter freed from the solubilizer (NaOH) by dialysis on a cellulose hydrate membrane.